1. Field of the Invention
The present invention relates to a process for obtaining a gas of a very low methane content but rich in hydrogen and carbon monoxide by continuous catalytic gasification of heavy oil having a specific gravity of higher than 0.7.
2. Description of Prior Art
As processes for the gasification of natural gas and light hydrocarbons of petroleum fractions up to naphtha, there have been known the partial oxidation process and the steam reforming process wherein a nickel catalyst is used, and the partial oxidation process wherein a catalyst is not used. On the other hand, for the gasification of hydrocarbons containing heavy distillates such as kerosene, gas oil and No. 2 fuel oil and heavy residues such as crude oil, atmospheric residue and vacuum residue, only the non-catalytic partial oxidation process is employed on an industrial scale.
A gas obtained by the known process wherein heavy oils such as heavy distillate and heavy residue are partially oxidized in the absence of catalyst has a low methane content since the reaction temperature in the partial oxidation is as high as 1300.degree.-1500.degree. C. and, therefore, it is used suitably as an ammonia, methanol or oxo synthesis gas or as hydrogen gas for hydrogenation. However, the non-catalytic partial oxidation process has the following defects:
(1) Expensive oxygen or oxygen-rich air is required in a large amount for maintaining a high reaction temperature. PA0 (2) A great part of the raw oil is spent for the combustion for obtaining a high temperature and, consequently, the yields of H.sub.2 and CO are reduced. PA0 (3) A reactor made of a heat-resistant material of a high grade is required because of the high reaction temperature and the life of the reactor is short. PA0 (4) Carbon deposition in an amount of 2-5% based on the raw material is unavoidable and, therefore, the yields of H.sub.2 and CO are reduced. Further, a superfluous cost of equipment is necessitated for the removal of carbon and recirculation into the raw material. This is economically disadvantageous. PA0 (1) The resultant gas has a low residual methane content and the yields of H.sub.2 and CO are high. Therefore, the gas is suitable for the production of synthesis gas to be used for the synthesis of ammonia and methanol. PA0 (2) When the gas is used for the synthesis of ammonia and methanol, the load due to the recycle of methane (inert component) in the synthesis reactor is small, since the methane content of the gas is extremely low. Accordingly, the power cost and capacity of the devices can be reduced economically advantageously. PA0 (3) The load for the separation of methane from H.sub.2 and CO can be reduced to also reduce the cost of the apparatus and energy. PA0 (1) Gasification efficiency is higher and yields of H.sub.2 and CO are higher, since the reaction temperature is lower by 200.degree.-600.degree. C. PA0 (2) Amount of expensive oxygen required is smaller. PA0 (3) Life of heat-resistant material of which the reactor is made is longer and heat-resistant materials of a high quality are not required. PA0 (4) By the use of the catalyst, carbon deposition is reduced in amount and, consequently, yields of H.sub.2 and CO are increased. Therefore, the resultant gas is suitable for the preparation of synthesis gas and a device for carbon recovery can be made smaller economically advantageously.
Various investigations have been made heretofore on the catalytic gasification of heavy oils for the purpose of overcoming said defects of the conventional processes for the partial oxidation of heavy oils. Recently, several processes have been reported, though they have not yet been put into practice on an industrial scale. Those processes mainly comprise contacting a heavy oil with a catalyst containing an alkali metal aluminate or calcium aluminate which is a composite oxide of an alkali metal or alkaline earth metal as the main component or a nickel catalyst containing a tungsten compound to gasify the heavy oil by steam reforming or partial oxidation.
Those processes have a merit that the gasification can be effected at a temperature as low as 800.degree.-1300.degree. C. Therefore, as compared with the non-catalytic partial oxidation process, those processes have the merits of a higher gasification efficiency and a smaller oxygen demand and, in addition, problems of materials of the reactor are less serious. Another merit of those processes is that the carbon deposition is small in amount and, therefore, the apparatus and cost required for the recovery of the carbon deposit are small.
However, after the gasification of the heavy oil in the presence of those catalysts, lower hydrocarbons (particularly methane) remain in a large amount in the resultant gas.
The presence of methane residue in the resultant gas is undesirable when the gas is to be used as a raw material for the synthesis of ammonia or methanol or as an oxo synthesis gas or as a source of hydrogen for hydrogenation, though the methane residue is preferred when the resultant gas is used as a fuel gas.
Thus, depending on the use of the resultant gas, the gas containing a large amount of residual methane is undesirable. The separation of methane residue from the resultant gas requires additional apparatus and energy. This is disadvantageous from both economical and industrial viewpoints.
It has been known that Fe, Co or Ni catalyst used in conventional processes has a remarkable effect of reducing the methane content. However, if this catalyst is used, carbon is easily deposited, thereby deteriorating the catalytic capacity and in case of a fixed bed system, the catalyst bed is sometimes clogged to make it impossible to continue the operation. Though it is effective for the prevention of carbon deposition to increase amount of steam, additional energy is required therefor uneconomically.
It is considered that the methane content of the resultant gas can be reduced by increasing the amount of steam, elevating the reaction temperature or elongating the residence time in the reactor. However, those ideas have a demerit of increasing the energy requirement and the cost of apparatuses. Further, it is difficult to reduce the methane content remarkably by those ideas.
Under circumstances as set forth above, there has been eagerly demanded the development of an economical process for obtaining a gas rich in hydrogen and carbon monoxide by the continuous catalytic gasification of heavy distillates such as kerosene, light oil and No. 2 fuel oil and heavy residue at a temperature as low as 800.degree.-1300.degree. C. substantially without forming hydrocarbon residue, particularly methane residue.